Polarization mitigation technique

ABSTRACT

A method and apparatus for mitigating the effects of polarization on wavelength determinations is disclosed. An optical source produces light across an optical spectrum, while a polarization element changes the polarization of the light at a first rate. The resulting light is applied to an optical element that produces a spectral response with a feature of interest from the polarization changed light. The optical element and the polarization element are such that the bandwidth of the feature of interest of the optical element is significantly greater than the first rate. A receiver network produces received signals from the received spectrum; and a data processing unit calculates a wavelength that is insensitive to ripple in the received signal and/or the received signals are low-pass filtered to reduce the ripple resulting from the polarization change.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 10/672,289 filed on Sep. 26, 2003 now U.S. Pat. No.7,173,696, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to measuring spectral responses of opticalelements. More particularly, this invention relates to improvingspectral response measurements by mitigating polarization dependence.

2. Description of the Related Art

Most optical components produce a spectral response that depends on thepolarization of incident light. This is often manifested as apolarization-dependent shift in the wavelength of a spectralcharacteristic (e.g. a reflection peak wavelength) of an opticalcomponent. If the incident light is highly polarized, this polarizationdependence can cause significant shifts in the wavelength of thespectral characteristic. In many systems, the polarization of theincident light is varying and unknown. This results in unpredictablyvarying shifts in the wavelengths of the spectral characteristic.

One of the many optical elements having polarization-dependent shifts inwavelength is the fiber Bragg grating (FBG) element. A fiber Bragggrating (FBG) element is an optical element that is usually formed byphoto-induced periodic modulation of the refractive index of an opticalfiber's core. An FBG element is highly reflective to light havingwavelengths within a narrow bandwidth that is centered at a wavelengththat is referred to as the Bragg wavelength. Other wavelengths arepassed without reflection. The Bragg wavelength itself is dependent onphysical parameters, such as temperature and strain, that impact on therefractive index. Therefore, FBG elements can be used as sensors tomeasure such parameters. After proper calibration, the Bragg wavelengthis an absolute measure of the physical parameters.

FBG sensors typically include a tunable laser that interrogates an FBGelement by sweeping across an optical spectrum that includes that Braggwavelength. Alternatively, a broadband light source/tunable filtercombination can be used. The sweeping light generates reflections fromthe FBG element that produce a spectral response of intensity verseswavelength. Since the spectral response peaks correspond to the Braggwavelengths of the FBG element, by determining the changes in responsepeaks produced by the physical parameter of interest that parameter canbe measured.

Unfortunately, in FBG sensors, the polarization-dependent wavelengthshift can limit the achievable measurement accuracy and resolution. Thisis because the spectral response peaks change not only because of thephysical parameter of interest, but also because of polarization inducedwavelength shifts.

That polarization-dependent wavelength shifts can impact measurements isknown, see reference Vines, Lasse, “Polarization Dependence in CHESSFiber Optic Strain Monitoring System Based on Fiber Bragg Gratings,”Norwegian Defense Research Establishment doc #: FFI/RAPPORT-2002/03348,ISBN-82-464-0645-0. That reference describes attempts at mitigating thepolarization problem by depolarizing the source radiation. However, whenusing narrow-bandwidth sources this puts stringent requirements on thedepolarizer design, often beyond what is practically achievable.

Therefore, a new method and apparatus of compensating forpolarization-dependent wavelength shifts would be beneficial.

SUMMARY OF THE INVENTION

The principles of the present invention enable compensation ofpolarization-dependent wavelength shifts in optical filter elements.

A polarization mitigated measurement apparatus that is in accord withthe present invention includes an optical source that scans across anoptical spectrum. That optical source is applied to a polarizationelement that changes the polarization of the output light. The output ofthe polarization element is applied to an optical filter element that issubject to polarization-dependent wavelength shifts. The bandwidth ofthe optical filter element is significantly greater than the rate ofchange of the polarization of the polarization element. The opticalfilter element produces a spectral response with an unknownpolarization-induced wavelength-shift transformation. A spectralmeasurement network measures that spectral response and produces areceived signal across the optical spectrum. A data processing unit thencalculates a filter wavelength that is insensitive to thepolarization-induced variations in the received signal across thebandwidth of the optical filter element. That calculated filterwavelength is subsequently used to characterize the optical filterelement.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 schematically illustrates an optical system that incorporates theprinciples of the present invention;

FIG. 2 illustrates both the spectral response and its dependence onpolarization of an FBG sensor;

FIG. 3 illustrates the use of a low-order (2nd) polynomial curve fit toproduce a measurement insensitive to polarization-induced variations;

FIG. 4 illustrates the spectral response of FIG. 2 both before and afterthat response is filtered; and

FIG. 5 schematically illustrates an FBG sensor system that incorporatesthe principles of the present invention;

To facilitate understanding, identical reference numerals have beenused, wherever possible, to designate identical elements that are commonto the figures.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Referring now to FIG. 1, a polarization mitigated apparatus 20 that isin accord with the present invention includes an optical source 24 thatscans across an optical spectrum. The optical source 24 is coupled to apolarization changing device 28 that changes the polarization of theoptical radiation (light) in a particular manner that is describedsubsequently. The output of the polarization changing device 28 isapplied to an optical element 32 that produces an unknown and/or varyingpolarization transformation on the optical radiation. The opticalradiation (light) is then applied to an optical element 72 having apolarization dependent wavelength response. The response of element 72has some spectral feature, or spectral features, of interest within theoptical spectrum produced by the optical source 24. The optical elements32 and 72 may also comprise a single element having the samefunctionality as described above.

The spectral bandwidth of the optical feature of interest in the opticalelement 72 is significantly greater than that of those induced by thechange of the polarization in the polarization changing device 28. Thatis, the polarization changing device 28 changes polarity quickly: such aquick change corresponds to a narrow spectral bandwidth.

FIG. 2 is an exemplary diagram 36 of the spectral response of theoptical element 72 to light from the system described above. Thepolarization dependency of the optical element 72 results in a spectralwaveform 40 of wavelength (in the X-axis 44) against normalizedamplitude (in the Y-axis 48). Curves 52 and 60 represent the spectralresponse of the feature of interest of element 72 for incident radiationon each of two orthogonal polarization axes. Waveform 40 represents theresponse of the feature of interest (Bragg wavelength) as a mixing ofthe two orthogonal polarization responses 52 and 60 due to thepolarization changing device 28. The diagram 36 might be a plot oftransmission through the optical element 72 or reflections from theoptical element 72.

In any event, the light from the optical element 72 is detected andconverted into electrical signals-by a receiver 64. The output of thereceiver 64 is applied through an optional low pass filter 68 (whosepurpose is described subsequently) to a data processor 76 that derives aparameter of interest, e.g., a Bragg wavelength, from the receiveroutput. A further, alternative arrangement is for a low pass filter tobe an integral part of the receiver 64.

The receiver 64 produces electrical signals that correspond to thespectral waveform 40. Thus, the waveform 40 also represents electricalsignals. In some embodiments of the present invention the data processor76 uses a noise insensitive peak detection method to characterize thewavelength of the spectral feature 40. Wherein such noise may berandomly produced or produced from a systematic source. An example ofsuch a technique in shown in FIG. 3 wherein the spectral feature 40A isbeing characterized. As shown, a quadratic curve 80 has been fit to themeasured signal 40A with a least-square error minimization technique.Examples of other suitable peak detection schemes include: center ofmass/centroid calculations, a fit of a polynomial curve, a Gaussiancurve, a Lorentzian curve, various power sine/cosine or othertrigonometric function curves, etc. Other curve fit methods may also beused. Regardless of the specific method used, in certain embodiments itis advantageous that the calculation of the wavelength corresponding tothe spectral feature is insensitive noise or variation at a frequencygreater than the bandwith of the spectral feature of interest.

Another approach to improving peak detection is to incorporate theoptional low-pass filter 68. That low-pass filter should have a cut-offfrequency that filters the higher frequency ripple content from theelectrical signal 40. This is shown, in FIG. 4, wherein the electricalwaveform 40A is smoothed by filtering to produce the waveform 90, whichhas reduced peaks and valleys. This filtering could be analogue ordigital, finite impulse response or infinite impulse response, or anycombination of these.

Is should be understood that a low pass filter, noise insensitive peakdetection techniques, or both can be used. Thus, low pass filtering andnoise insensitive peak detection techniques are complementary ratherthan mutually exclusive.

Turning back to FIG. 1, the optical source 24 could be any of a numberof devices such as a tunable laser or a combination of a broadband lightsource and a tunable filter. The polarization changing device 28 can bea depolarizer or a polarization scrambler, either active or passive. Itcould also be integrated into the source 24, or even be an inherentproperty of the source 24. Commonly used passive depolarizers includewedge depolarizers, cascaded feed-back loop depolarizers and Lyotdepolarizers. Commonly used polarization scramblers include LithiumNiobate, resonant coil or fiber squeezer based devices.

The optical element 32 that produces the unknown polarizationtransformation can be any of a wide range of optical elements. Indeed,most optical elements show polarization dependence in their transmissionor reflection spectral responses. An example of such an optical elementis the fiber optic cable. Not only do fiber optic cables showpolarization dependence, but fiber optic cables can incorporate fiberBragg grating elements that also exhibit polarization dependence. Aspreviously noted, fiber Bragg grating elements can be used in FBG sensorsystems.

FIG. 5 illustrates a specific embodiment of the invention, specificallyan FBG sensor system 118 having FBG elements 120 within an FBG sensorarray 130. As shown, the FBG sensor array 130 may be comprised of one ormore optical fibers 145 and 147. The individual FBG elements 120 haveBragg wavelengths λ1 through λ5. As noted, Bragg wavelengths aredependent upon physical parameters such as temperature and pressure, andthus changes in wavelengths λ1-λ5 can be made indicative of parametersbeing sensed. Unfortunately, measurement errors caused by polarizationshifts in the optical fibers and other optical elements can reduce theaccuracy.

The FBG sensor system 118 is suitable for measuring pressure andtemperature in hostile environments such as occurs in oil wells. Toprovide a reference wavelength the FBG sensor system 118 includes anoptical fiber 149 having a reference FBG element 160 that is physicallyand thermally protected by an enclosure 162. The reference FBG element160 is comprised of a fiber Bragg grating 166 that is induced in thecore of the optical fiber 149. When light is applied to the referenceFBG element 160 reflections of light at Bragg wavelengths are produced.The enclosure 162 protects the reference FBG element 160 such that itsBragg wavelengths are not susceptible to external influences and thusare accurately known. Alternatively, a thermometer could be used todetermine the temperature of the reference FBG element 160. Then, basedon the measured temperature the Bragg wavelengths of the FBG element 160could be temperature compensated. Either way, the reference FBG element160 produces Bragg wavelengths that are accurately known and that can beused to process other Bragg wavelengths. Any measurement error in theBragg wavelength of the reference FBG element 160 caused by polarizationshifts in the optical fiber 149 or in other optical elements will reducethe accuracy of all of the Bragg wavelength measurements.

The FBG sensor system 118 further includes a tuned laser 140 that isscanned across the Bragg wavelengths of the FBG elements 120 and of thereference FBG element 160. The tuned laser 140 corresponds to the source24 of FIG. 1. The narrow bandwidth scanning light is applied to apolarization changing device 28 (also see FIG. 1 and the foregoingdescription of the polarization changing device 28), which genericallyrepresents polarization changes induced by the tuned laser 140 and bythe optical transmission path. Other optical elements, including theoptical fibers 145, 147, and 149, can also induce polarization changes.

The output of the polarization changing device 28 is split by a fiberoptic directional coupler 180. The main portion of that light is coupledto the FBG sensor array 130 and to the reference FBG element 160 via asecond directional coupler 202. Reflected light from the FBG sensorarray 130 and from the FBG element 160, which occur when the wavelengthof the narrow bandwidth scanning light sweeps across the Braggwavelength of an FBG element 120 or of the reference FBG element 160,passes back into the directional coupler 202 and into a sensor receivernetwork 219. That receiver includes a sensor detector 220 that convertsthe Bragg wavelength reflections into sensor electrical signals havingamplitudes that depend on the power (intensity) of the reflected light.Thus, the sensor detector 220 acts as a power meter.

The output of the sensor detector 220 is applied to a sensor electricalfilter 226 (which corresponds to the filter 68 of FIG. 1), which is partof the sensor receiver 219. The sensor electrical filter 226 low-passfilters the sensor electrical signals to reduce the polarizationdependent portions of the electrical signals. The polarization dependentportions can be caused by polarization changes induced by thepolarization changing device 28 and by the optical fibers 145, 147, or149.

The output of the sensor electrical filter 226 is applied to a sensoramplifier 228, which is also part of the sensor receiver 219. The sensoramplifier 228 amplifies the output of the sensor electrical filter 226.Alternatively, the sensor amplifier 228 could come before the sensorelectrical filter 226, or sensor electrical filters 226 can be placedboth before and after the sensor amplifier 228.

A portion of the light from the fiber optic directional coupler 180 isdirected along a reference arm 250 having an interference filter 326,which is, for example, a fixed cavity F-P fiber filter. The interferencefilter 326 produces a reference spectrum having spectrum peaks with aconstant, known frequency separation that depends on the interferencefilter 326. The reference spectrum is coupled to a reference receiver329. The reference receiver 329 includes a reference detector 330 thatproduces a reference electrical pulse train that corresponds to theoutput of the interference filter 326. The output of the referencedetector 330 is filtered by a reference filter 332, which is part of thereference receiver 329. The reference filter 332 low pass filters thereference electrical signals to reduce the polarization dependentportions of the electrical signals. The output of the reference filter332 is applied to a reference amplifier 336, which is also part of thereference receiver 329, and which amplifies the output of the referencefilter 332. Alternatively, the reference amplifier 336 could come beforethe reference filter 332.

Once the wavelength of one of the reference spectrum peaks is known,because of the constant frequency separation produced by theinterference filter 326 all of the wavelengths of the peaks can bedetermined. Then, by comparing the Bragg wavelengths of the FBG elements120 to the wavelengths of the reference spectrum peaks the Braggwavelengths of the FBG elements can be accurately determined.Furthermore, since the unstressed Bragg wavelengths of the FBG elements120 are known, the wavelength change in each FBG element's Braggwavelength can be used to determine a physical parameter of interest.

To that end, the electrical signals from the sensor receiver 219 andfrom the reference receiver 329 are sequentially sampled, processed andcompared in a signal processing and data presentation unit 366 toproduce such measurements. That unit interrogates the referenceelectrical signals to isolate the responses from the reference FBGelement 160 (which are different than the wavelengths λ1 through λ5).Those responses are then processed as is described below to produce acharacteristic wavelength of the reference FBG element 160. Thatcharacteristic wavelength is then used to identify at least onereference peak, which together with the known reference peak spacing,are used as to determine the Bragg wavelengths λ1 through λ5.

A key to accurately determining Bragg wavelengths λ1 through λ5 isaccurately determining the characteristic Bragg wavelength of thereference FBG element 160. To determine that Bragg wavelength the signalprocessing and data presentation unit 366 performs a mathematicalanalysis of the reference electrical signals and of the sensorelectrical signals to reduce polarization induced measurement errors.That analysis uses signal processing techniques that are insensitive toand/or that remove unwanted noise and fluctuations in the receivedsignals from both the reference receiver 329 and from the sensorreceiver 219. The analysis can include mathematical techniques such asfitting quadratic curves to the electrical signals using a least-squareerror minimization technique as described above. Of course, othermathematical techniques could be used. Furthermore, the low-passfiltering of the reference electrical signals and of the sensorelectrical signals also reduce polarization induced measurement errors.By compensating for polarization induced changes the measurementaccuracy can be improved.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method to compensate for polarization fluctuation, comprising:interrogating a sensor with light having a varying polarization toproduce a response signal, wherein the polarization varies at a firstrate; and analyzing the response signal at a second rate greater thanthe first rate, wherein the analyzing occurs throughout the varyingpolarization applied during the interrogating such that data formultiple different polarizations is utilized to determine an outputinsensitive to polarization fluctuations.
 2. The method of claim 1wherein the light is within an optical spectrum.
 3. The method of claim1 wherein the response signal conveys a physical parameter being sensedthat varies with the polarization changes.
 4. The method of claim 1wherein analyzing the signals includes low-pass filtering the signals.5. The method of claim 1 wherein the output is a wavelength.
 6. Anapparatus to determine a polarization fluctuation insensitive output,comprising: a light source; a polarization element coupled to the lightsource which varies the polarization of light from the light source,wherein the polarization element alters the polarization of the light ata first rate; a sensor which when interrogated with light having varyingpolarizations from the polarization element produces a response signal;and a processing unit that analyzes the response signal from the sensorand determines an output that is insensitive to polarizationfluctuation, wherein the processing unit analyzes the response signal ata second rate greater than the first rate.
 7. The apparatus of claim 6wherein the light source produces light across an optical spectrum. 8.The apparatus of claim 6 wherein the sensor includes a Bragg grating. 9.The apparatus of claim 6 wherein the sensor includes a receiver whichconverts a spectral response into the response signal.
 10. The apparatusof claim 9 wherein the receiver includes a low-pass filter.
 11. Theapparatus of claim 6 wherein the output is a wavelength.